Voltage-controlled oscillators (VCO) are one of the most essential building blocks in phase-locked loops and frequency synthesizers, which are required in all data and telecommunication communication systems to generate highly accurate timing reference sources.
Typical oscillators use inductors and capacitors as resonant tars for signal oscillation. Active circuits compensate for any loss in the tank and sustain the oscillation. For an ideal case, the oscillation signal is a pure tone (FIG. 1a). However, noise in the circuit is converted into phase noise, which is exhibited as a roll-off skirt in the frequency spectrum (FIG. 1b). This phase noise determines the frequency purity of the oscillating source and significantly affects the signal-to-noise and bit-error-rate performance of the whole system.
Phase noise follows the classical Leeson-Cutler model
      S    SSB    =      F    ⁢          kT              2        ⁢                  P          sig                      ⁢                  w        σ        2                              Q          2                ⁢        Δ        ⁢                                  ⁢                  ω          2                    where F is an empirical parameter that quantifies how effectively the noise is converted into phase noise and Psig is the power of the oscillating signal.
To minimize the phase noise, the conversion factor F of the noise to the phase noise needs to be minimized while the signal power needs to be maximized.
The basic mechanism behind the generation of phase noise can be understood by considering a simple LC tank with impulse current noise. In FIG. 2(a), if the noise impulse is applied at the peak of the output voltage, only the voltage amplitude will be changed, and current noise is only converted to amplitude noise. On the other hand, as shown in FIG. 2(b), if the noise impulse is applied at a zero-crossing point of the output waveform, the voltage is time shifted. This is equivalent to an instantaneous change in oscillation frequency. In this case, current noise is converted to phase noise.
This observation demonstrates that careful positioning and aligning of the noise injection relative to the output voltage waveform can help to achieve the lowest possible phase noise conversion.
For an oscillator with a signal current I and a tank impedance Z, the output amplitude is simply given by V=I×Z. To maximize the output amplitude, either I or Z can be increased. However, increasing current would result in an increase in power consumption, which is not desirable in many wireless and portable applications.
Alternatively, the impedances Z can be increased to increase the output amplitude. Consider a simple LC tank with the inductor's series resistance in FIG. 3. As illustrated in the figure, for a fixed oscillation frequency, maximizing L and minimizing C can be beneficial. Firstly, the tank impedance increases at the center frequency, which gives larger signal amplitude for a given bias current. This minimizes phase noise and maximizes current efficiency. Secondly, the filtering capability of the tank (or quality factor Q) increases, which can help filtering out the random frequency fluctuation and again minimizes phase noise.
As the tank impedance increases with L, the loop gain also increases and gives better startup reliability of the oscillator.
A large L/C ratio, however, gives a smaller frequency tuning range of the oscillator because the tuning of the capacitance C is limited to a narrower range. The following table summarises the options depending on the desired objective.
LOW-POWER LOW-PHASE-NOISE OPTIMIZATION SUMMARYlow powerlow phasenoiseLmaximizemaximizeCminimizeminimizeRminimizeminimizeAmplitudeminimizemaximize